1. Field of the Invention
The present invention relates to form-in-place fluoropolymer gasketing elements which exhibit excellent creep relaxation and dimensional stability when subjected to compressive loading.
2. Description of Related Art
Polytetrafluoroethylene (PTFE) has demonstrated utility in many areas. As an industrial material, such as a gasketing material, for example, PTFE has exhibited excellent utility in harsh chemical environments, which normally degrade many conventional metals and polymeric materials. PTFE is also usable over a broad temperature range, from as high as 260.degree. C. to as low as near -273.degree. C.
However, conventional non-porous PTFE materials, which are typically compression molded or extruded, exhibit poor mechanical properties, such as low tensile strength and low resistance to creep relaxation. Creep relaxation, particularly that experienced under compression, is a particular problem for conventional PTFE sealants, which materials have a tendency to spread out under compressive load, thereby reducing their effective sealability over time. Accordingly, despite a number of highly desirable properties, use of PTFE is generally limited to situations not requiring high tensile strength or good resistance to compressive creep relaxation.
Polytetrafluoroethylene may be produced in a porous, expanded form as taught in U.S. Pat. No. 3,953,566 to Gore. This processing creates orientation of the material, and correspondingly strength, primarily in just the longitudinal direction; however biaxial expansion and expansion in the longitudinal direction is also possible. This material is characterized by a plurality of nodes interconnected by fibrils which are oriented based on the direction(s) of expansion. This material may be formed in the shape of films, tubes, rods and continuous filaments. Thus, porous expanded polytetrafluoroethylene (ePTFE) is of a higher strength than unexpanded forms of PTFE, has the chemical inertness of conventional PTFE, and when used as a gasket, has an even wider temperature range of up to as high as 315.degree. C.
Examples of porous, expanded polytetrafluoroethylene gasket materials include GORE-TEX.RTM. Joint Sealant and GORE-TEX.RTM. Gasket Tape(manufactured by W. L. Gore & Associates, Inc., Elkton Md.), which are available as continuous (i.e., greater than 15 m in length), "form-in-place" sealants. These continuous, form-in-place, conformable ePTFE gaskets generally have their strength oriented in the longitudinal direction of the material due to the expansion of the PTFE in only the longitudinal direction. Thus, the resistance to creep relaxation is primarily in just the longitudinal direction. Such porous ePTFE joint sealants and gasket tapes provide excellent seals in applications having relatively wide sealing surfaces and where sufficient clamping loads can be applied to them so that, as the sealant compresses between the sealing surfaces to form a thin, wide gasket, the level of compressive stress is sufficient to densify the gasket and to provide the desired sealability. Consequently, these gaskets are typically not well suited to applications having narrow sealing surfaces or requiring relatively thick gaskets.
For optimum sealing performance in a gasket, resistance to creep relaxation is desired in both the longitudinal and transverse directions of the material. Materials which exhibit resistance to creep relaxation in multiple directions are commercially available in sheets of bi-axially expanded PTFE, limited to discrete lengths and widths, from which gaskets may be cut to fit a desired geometry (and which is commercially available as GORE-TEX GR.RTM. sheet gasketing.
W. L. Gore & Associates, Inc., has produced a number of continuous, form-in-place ePTFE sealants with significant longitudinal and transverse strength. The first consists of a porous polytetrafluoroethylene core, expanded in the longitudinal direction and helically wrapped with a longitudinally expanded sheet, thus imparting transverse strength to the composite by means of the helical wrap, creating a sealing cord of significant longitudinal and transverse strength. This material is commercially available as GORE-TEX.RTM. gasket material for use as a static seal between multiple plates of plate-and-frame heat exchangers and plate-and-frame filters. A second, continuous, form-in-place, expanded polytetrafluoroethylene seal with significant multidirectional strength is commercially available as GORE-TEX.RTM. Valve Stem Packing (manufactured by W. L. Gore & Associates, Inc., Elkton Md.). This material consists of a core of twisted, bi-axially expanded polytetrafluoroethylene membrane which is helically wrapped with a longitudinally expanded PTFE sheet.
All the above-mentioned variations for imparting strength in the longitudinal and transverse direction, and therefore resistance to creep relaxation in multiple directions, produce continuous, form-in-place sealing elements in the form of helically wrapped cords, tubes or similar geometries which may be formed by a helical-wrap formation technique. Although these materials work quite well, it is believed that even further improvements may be possible, for example, for ease in processing and to achieve other properties desired for gasketing elements.
Accordingly, it is a primary purpose of the present invention to provide a continuous, dimensionally stable, form-in-place gasketing element that is resistant to creep relaxation in both the longitudinal and transverse directions and which does not require the complicated manufacturing set-ups associated with helical-wrap formation techniques.